1. Field of the Invention
The present invention relates to reduction of metal oxides in an electrolytic cell.
2. Description of Related Art
The present invention was made during the course of an on-going research project on the electrolytic reduction of titania (TiO2) carried out by the applicant.
During the course of the research project the applicant carried out experimental work on an electrolytic cell that included a graphite crucible that formed an anode of the cell, a pool of molten CaCl2-based electrolyte in the crucible, and a cathode that included solid titania.
One objective of the experimental work was to reproduce the results reported in International application PCT/GB99/01781 (Publication no. WO99/64638) in the name of Cambridge University Technical Services Limited and in technical papers published by the inventors.
The Cambridge International application discloses two potential applications of a “discovery” in the field of metallurgical electrochemistry.
One application is the direct production of a metal from a metal oxide.
In the context of this application, the “discovery” is the realisation that an electrolytic cell can be used to ionise oxygen contained in a metal oxide so that the oxygen dissolves in an electrolyte. The Cambridge International application discloses that when a suitable potential is applied to an electrolytic cell with a metal oxide as a cathode, a reaction occurs whereby oxygen is ionised and is subsequently able to dissolve in the electrolyte of the cell.
European patent application 9995507.1 derived from the Cambridge International application has been allowed by the European Patent Office.
The allowed claims of the European patent application inter alia define a method of electrolytically reducing a metal oxide (such as titania) that includes operating an electrolytic cell at a potential that is lower than the deposition potential of cations in the electrolyte.
The Cambridge European patent application does not define what is meant by deposition potential and does not include any specific examples that provide values of the deposition potential for particular cations.
However, submissions dated 2 Oct. 2001 to the European Patent Office by the Cambridge patent attorneys, which pre-dated the lodgement of the claims that were ultimately allowed, indicate that they believe that the decomposition potential of an electrolyte is the deposition potential of a cation in the electrolyte.
Specifically, page 5 of the submissions state that: “The second advantage described above is achieved in part through carrying out the claimed invention below the decomposition potential of the electrolyte. If higher potentials are used then, as noted in D1 and D2, the cation in the electrolyte deposits on the metal or semi-metal compound. In the example of D1, this leads to calcium deposition and therefore consumption of this reactive metal . . . During operation of the method, the electrolytic cation is not deposited on the cathode”.
Contrary to the findings of Cambridge, the experimental work carried out by the applicant has established that it is essential that the electrolytic cell be operated at a potential that is above the potential at which Ca++ cations in the electrolyte can deposit as Ca metal on the cathode.